As the automotive industry has moved toward cleaner and more fuel-efficient products, engine compartments have become packed with engines that run hotter, with materials that absorb less heat (aluminum vs. steel), and with materials that deform or melt when exposed to excessive heat. In addition, other portions of automobiles include components that generate significant heat, e.g., catalytic heaters. To address this problem, simple metal stampings have been positioned between the heat source and the component to be protected. As more components are packed into less space, heat generation has increased to the point (e.g., over 1300.degree. F. at a catalytic converter) where more sophisticated thermal shields are required. Additionally, as automobile manufacturers continuously strive to reduce noise within the passenger compartment of automobiles they produce, simple stampings, with their tendency to vibrate, are inadequate for many applications.
To address these concerns, composite thermal shields are now in widespread use. These thermal shields function by reflecting, deflecting, dispersing and/or absorbing heat. Generally, these thermal shields include an insulating material, e.g., fiberglass, ceramic, aramid or air, that is typically encapsulated by upper and lower plates made from stainless steel, aluminum or other materials of varying grades and thicknesses. Such composite thermal shields are described, for example, in U.S. Pat. Nos. 2,576,698 and 5,398,407. Factors such as size and temperature of the heat source, air flow, ambient temperature and the required temperature on the cool side of the shield, are considered in designing composite thermal shields and typically necessitate an application-specific design.
Known composite thermal shields suffer from a number of problems. First, the seam at the periphery of the thermal shield used to secure together the upper and lower plates, which is generally formed by folding together peripheral portions of the plates, is typically not waterproof. As a result water can enter the interior of the thermal shield. The presence of water in the thermal shield reduces insulting properties, and can lead to corrosion, thereby reducing the durability of the thermal shield. Furthermore, the peripheral seams of known thermal shields are often wrinkled and cracked which renders them less desirable to consumers and in extreme cases necessitates scrapping the part. In addition, such known peripheral seams are often sharp, causing safety issues in the workplace.
Another problem with known composite thermal shields is that they are relatively costly to manufacture. The seams of known thermal shields could be made waterproof by welding, through the use of adhesives or by other techniques. However, the additional manufacturing steps associated with these processes would add further to the cost of producing a thermal shield, which is already often higher than is desired.